1. Field of the Invention
The invention relates to a method and apparatus for shipboard regasification of liquefied natural gas on liquefied natural gas (“LNG”) carriers, not fitted with steam propulsion plants. In particular, this invention relates to using the thermal energy of a propulsion system for a LNG carrier, such as a diesel engine or gas turbine propulsion plant which ordinarily drives the propeller shaft and propeller of a ship, to serve an additional function of providing heat to a shipboard regasification system.
2. Description of the Related Art
Conventional steam propulsion plants of sea-going vessels often have two main boilers providing high pressure superheated steam to cross compound steam turbines driving a single shaft line and propeller through double reduction gears. Many of these vessels are liquefied natural gas carriers. Steam has been a popular choice for propulsion plants for liquefied natural gas carriers, primarily due to the ease of burning the boil-off gas from the LNG cargo containment system. When the conventional steam propelled LNG carrier is fitted with regasification equipment, the main steam boilers of the conventional steam propulsion plant served, to provide both high-pressure superheated steam to drive a propeller and propeller shaft of the liquefied natural gas carrier vessels as well as a natural source of heat for regasification of liquid natural gas. Heat from the vessel's steam propulsion plant acts as a primary heat source, with an upgrade in the output of the boilers to match the desired regasified liquid natural gas sendout rate.
Although the steam propulsion plant provides a natural source of heat for shipboard regasification and a simple method for burning of boil-off gas, it is very inefficient thermal cycle for propelling a ship, as compared to modern diesel engines or advanced gas turbine cycles. By contrast, the diesel or gas turbine engines do not provide a comparable amount of available thermal energy to satisfy shipboard regasification, which requires significant heat to gasify the liquefied natural gas prior to its discharge to the shore.
Because of the inefficiency of steam turbine propulsion plants and the current trend to alternate propulsion plants for LNG carriers, the present invention has been developed to use a more efficient propulsion plant such as a diesel engine or gas turbine. The more efficient diesel engine and gas turbine propulsion plants will either provide direct mechanical drive of the propeller and propeller shaft or will be fitted with an integrated electric power plant. However, this alternative propulsion arrangement eliminates the vessel's main steam boilers, which also served as the natural heat source for shipboard regasification. Therefore, there is a need to overcome the lack of a readily available heat source for shipboard regasification in diesel engine and gas turbine propulsion plants.